\documentclass{standalone}
\usepackage{tikz}
\usepackage{ctex,siunitx}
\setCJKmainfont{Noto Serif CJK SC}
\usepackage{tkz-euclide}
\usepackage{amsmath}
\usepackage{wasysym}
\usetikzlibrary{patterns, calc}
\usetikzlibrary {decorations.pathmorphing, decorations.pathreplacing, decorations.shapes,}
\begin{document}
\small
\begin{tikzpicture}[>=latex,xscale=0.6]
  % \useasboundingbox(0.9,0)rectangle(5.1,5);
  \draw [->](-2,0)--(11,0)node[right]{$\omega t$};
  \draw [->](0,-1.5)--(0,1.5)node[right]{$i,u$};
  \draw [ultra thick] plot[domain=0:3*3.1416, samples=1000] function{sin(x)} node [above]{$u$};
  \draw [very thick] plot[domain=0:3*3.1416, samples=1000] function{.8*sin(x+3.1416/2)} node [below]{$i$};
  \draw [very thick, dashed] plot[domain=-3.1416/2:0, samples=100] function{.8*sin(x+3.1416/2)};
  %\draw [ultra thick, dashed] plot[domain=-1:0, samples=100] function{sin(x+1)};
  \node at (-.25,-.25){$O$};
  
  \foreach \x in {1,5}
  {
      \draw[dashed] (\x*3.1416/2,0)--(\x*3.1416/2,1);
      \draw[dashed] (\x*3.1416/2+3.1416/2,0)--(\x*3.1416/2+3.1416/2,-.8);
  }
  
  \draw[dashed] (4*3.1416/2,0)--(4*3.1416/2,.8);
      \draw[dashed] (5*3.1416/2,0)--(5*3.1416/2,1);
      \draw[dashed] (3*3.1416/2,0)--(3*3.1416/2,-1);
\end{tikzpicture}
\end{document}